At the present time, cathode ray tubes are the most frequently used electronic display devices for applications such as computers and television picture displays. Cathode rays tubes have been very satisfactorily developed for these purposes. However, they continue to have serious disadvantages, including shortcomings in the clarity and definition of the displayed information, lack of flatness in the display screen, bulkiness and heavy weight, and relatively high cost and high energy consumption.
It has been long recognized that if liquid crystal display devices could be developed to serve for these purposes, a number of the disadvantages enumerated above could be overcome. Many attempts have been made to provide large high definition liquid crystal displays. A number of these methods and the resulting structures are described in some detail in a chapter entitled "Liquid Crystal Matrix Displays" by Eiji Kameko in a book entitled "Image Pickup and Display" edited by B. Kazan and published by Academic Press in New York, N.Y. in 1981.
As explained in the above-mentioned reference, it is difficult to build matrix addressed liquid crystal displays with high capacity and high resolution because of the dynamic properties of liquid crystal materials. The individual cells of such a matrix forming individual pixels have to be written sequentially. The more pixels the array has, the less write time per pixel is allowed. To switch the liquid crystal material from one orientation to another in these short times requires a high voltage, which, however, disturbs non-addressed pixels. Accordingly, it is difficult to achieve precise voltage control on all of the pixels in a large display without providing other associated non-linear devices to operate as gating devices in connection with the matrix. Prior proposals for solution of this problem have included the addition of a suitable non-linear element to each liquid crystal display pixel of an array. Various non-linear elements have been proposed including transistors, diodes, and varistors.
One important prior approach has been to use thin film transistors. See U.S. Pat. No. 3,824,003 Koda et al. and U.S. Pat. No. 4,239,346 Lloyd. However, the physical size of such a display is substantially limited because, in order to obtain an adequate yield, with reproducability, and reliability, it is necessary to produce the transistor array on a single crystal of silicon. Large single crystal wafers are currently not available, and would be very expensive if they were available.
Another disadvantage with the transistors is that most of the presently known liquid crystal display materials should be subjected to frequent voltage reversals in order to prolong the life of the material, and the transistor matrices become much more complex if capable of operating in this fashion.
Another promising approach has been to use varistors, which are based upon a thick ceramic plate of a zinc oxide mixture. However, the operating voltage required is rather high, being on the order of 60 volts, and large plates are difficult to reproduce accurately, grain size is relatively large, and cycling behavior is questionable.
Another problem with liquid crystal display matrix structures has been that the display must frequently be renewed. Liquid crystal display pixel cells are essentially small capacitors which will hold a charge, and a display state, if the associated driving circuit does not have the characteristic of rapidly dissipating that charge. Prior matrix structures have often permitted a too-rapid charge dissipation.
Accordingly, it is one object of the present invention to provide an improved liquid crystal display matrix structure in which display refresh information need not be supplied with great frequency because the driving circuits for the display pixel cells do not readily permit the charge of the cells to leak off.
Another object of the invention is to provide an improved liquid crystal display structure which is characterized by a sharp switching operation from one display node to another.
Another object of the invention is to provide an improved liquid crystal display structure which is adapted for the production of very large displays having many pixels and high graphic definition.
Another object of the invention is to provide an improved liquid crystal display structure which is capable of reliable low voltage operation.
Another object of the invention is to provide an improved liquid crystal display structure which is characterized by relatively low cost.
Further objects and advantages of the invention will be apparent from the following description and the accompanying drawings.